Tropospheric ozone during Mauna Loa observatory photochemistry experiment 2 compared to long-term measurements from surface and ozonesonde observations

Continuous surface ozone measurements have been made at Mama Loa Observatory (MLO) for 20 years. In addition, weekly ozone profile measurements using balloonborne ozonesondes have been carried out from Hilo, Hawaii, since 1985. These long-term records are compared with data obtained during the MLOPEX 2 period from September 1991 to August 1992. Ozone behavior at the observatory level (similar to 3.4 km) during autumn and winter of 1991-1992 was similar to that found during the period 1980-1990. In spring and summer 1992, however, there were several significant differences from the long-term behavior. During March and April 1992, there was about 10% more ozone than the long-term average, and the variability was less than half of what is seen normally. These characteristics are associated with strong flow from the north and west. Both June and July 1992 saw periods of elevated ozone with the June average 20% higher than normal. During the more limited sampling (weekly profiles) when ozonesonde measurements were made, the 1992 spring enhancement was particularly pronounced at 500 mbar (similar to 6 km), while during the summer the larger than normal concentrations were at 700 mbar (similar to 3.5 km). In the upper troposphere, on the other hand, spring ozone amounts in 1992 were much below normal with only about half the ozone usually seen in the 12- to 15-km region. The ozone profiles are discussed in terms of the representativeness of the MLO surface measurements in characterizing the free troposphere ozone behavior at both the altitude of the observatory as well as other heights in the atmosphere. During the winter and spring, the MLO measurements are often representative of behavior over a broad depth of the troposphere (3-10 km). In the summer and autumn the MLO observations are more characteristic of free tropospheric conditions at or near the observatory level.